Accompanying with increasing sharp contradiction between outbreak of users' demands for high-bandwidth wireless services and scarce spectrum resources, mobile operators start to consider taking unlicensed frequency bands as a supplement for licensed frequency bands. Thus, research about deploying long term evolution (LTE) on unlicensed frequency bands has been scheduled. Third generation partnership project (3GPP) starts to research and pass effective carrier aggregation of unlicensed frequency bands and licensed frequency bands. As shown in FIG. 1(a) and FIG. 1(b), when guaranteeing that no obvious effect is generated for other technologies of unlicensed frequency bands, how to effectively improve utilization of entire network's spectrum is an urgent technical problem to be solved.
Unlicensed frequency bands have generally been allocated for other applications, e.g., wireless fidelity (WiFi) of radar or 802.11 series. Thus, interference level on unlicensed frequency bands is uncertain. Subsequently, it is generally difficulty to guarantee quality of service (QoS) of LTE transmission. However, the unlicensed frequency bands may still be applied for data transmission with lower QoS requirements. Here, an LTE system deployed on the unlicensed frequency bands may be referred to as an LTE-unlicensed (LTE-U) system. How to avoid mutual interference between the LTE-U system and other wireless systems, such as radar, or WiFi, on the unlicensed frequency bands is a key problem.
Clear channel assessment (CCA), which is a mechanism to avoid collision, is generally employed by the unlicensed frequency bands. Before transmitting a signal, a station (STA) must detect a wireless channel. When detecting that the wireless channel is idle, the STA may occupy the wireless channel and transmit the signal. The LTE-U system also needs to comply with a similar mechanism, so as to ensure a small interference to other signals. A simpler method is as follows. An LTE-U device (base station or terminal user) may be dynamically opened or closed, based on a CCA result. That is, when detecting that a channel is idle, the LTE-U device may transmit a signal. When detecting that a channel is busy, the LTE-U device may not transmit a signal.
In the current LTE system, random access process may be executed by a primary cell (Pcell) or a secondary cell (Scell).
The objective for executing the random access process by the Pcell may be as follows. Establish an initial connection for a UE in a radio resource control_idle (RRC_IDLE) state. Re-establish an RRC connection for a UE. Switch a cell. A downlink service arrives, or an uplink service arrives, when a UE in the RRC connected state is out-of-step in an uplink direction. Locate a UE in the RRC connected state.
The random access process may be executed by the Scell, so as to establish uplink synchronization on a corresponding secondary timing advance group (sTAG).
The random access process for implementing foregoing uplink synchronization may include three blocks as follows.
A first block: a base station may allocate a random access preamble by using downlink signaling.
The base station may allocate a non-competitive random access preamble for a user.
The downlink signaling may be borne by a physical downlink control channel (PDCCH) order or other high-layer signaling.
The PDCCH order may include a preamble index and a mask index of a physical random access channel (PRACH) channel.
A second block: a user may transmit the random access preamble on the PRACH channel.
The random access preamble may be transmitted by the user, based on code word sequence and time-frequency resources indicated by the signaling received in the first block.
In the first PRACH resource subframe when 6 ms is passed after receiving the PDCCH order, the UE may transmit the random access preamble on an uplink carrier scheduled by the PDCCH order through a PRACH channel. That is, suppose the PDCCH order is received in subframe n, PRACH may be transmitted in the first PRACH resource subframe after subframe n+k (k>=6).
A third block: the base station may transmit a random access response born by a physical downlink shared channel (PDSCH).
The UE may receive the random access response within a random access response window. When no random access response is received, the UE may make preparations for re-transmitting the PRACH within 4 ms after the end of the random access response window.
In the current LTE system, various timing advance groups (TAGs) may be configured for a UE. Each TAG may include at least one carrier. Based on the foregoing blocks, a base station may establish uplink synchronization for each TAG. When all the carries within a TAG belong to unlicensed frequency bands, a UE needs to perform the CCA detection to a carrier triggered by the PDCCH order. The UE may transmit the PRACH, only when the carrier is idle. Because of different geographic locations of base station and UE, and difference between scheduling time and actual transmitting time, an uplink carrier channel, the PRACH transmission of which is triggered by the base station, probably be occupied, and then the UE may not execute uplink transmission. Subsequently, uplink synchronization delay may be increased.
It should be noted that, the foregoing descriptions about technical background are put forward, so as to provide a clear and complete description for technical solutions of the present disclosure, and to facilitate understandings of persons having ordinary skill in the art. It is not appropriate to consider that the foregoing technical solutions are well known to persons having ordinary skill in the art, only because the foregoing technical solutions are described in background section of the present disclosure.